you should really use 14.4v for your calculations on the alternator. a battery alone would be 12v. so, your calculations should be I=P/E, so 620/14.4 = 43.05 amps. I'm pretty sure that all you accessories are LED's and they have internal capacitors to help power them, so you should not be able to stress the alternator. if you are really worried about it then go to a auto stereo store and get a big holding capacitor and wire it into your electrical system, it will boost your system and not let the voltage drop when you put a sudden load on it. usually 80% of service factor is what you would want to run a alternator, ie generator, transformer or other electrical equipment.